Physical units with pint

[xoolive]

It would probably make sense to annotate each column of the pandas dataframe with physical units. A basic improvement could be a basic annotation for features; a more advanced improvement would check consistency and convert units if need be.

Reference:

• Pint https://pint.readthedocs.io/en/latest/

[xoolive]

The library has now a pandas extension: https://github.com/hgrecco/pint-pandas.
We should have a look together with @junzis in https://github.com/junzis/openap to avoid potential troubles.

This could lead some incompatibility issues with past files, so maybe data without unit should be set to standard unit (e.g. feet for altitudes, etc.)